Abdominal Aortic Aneurysms

Patient professional reference

Professional Reference articles are written by UK doctors and are based on research evidence, UK and European Guidelines.
They are designed for health professionals to use. You may find the Abdominal Aortic Aneurysm article more useful, or one of our other health articles.

Definition

An aneurysm is a permanent and irreversible dilatation of a blood vessel by at least 50% of the normal expected diameter.[1]Aortic aneurysms are classified as abdominal (the majority) or thoracic.

An aneurysm is caused by degradation of the elastic lamellae, a leukocytic infiltrate, enhanced proteolysis and smooth muscle cell loss. The dilatation affects all three layers of the arterial wall. A false aneurysm (pseudoaneurysm) is caused by blood leaking through the arterial wall but contained by the adventitia or surrounding perivascular tissue.

The 'normal' diameter of the abdominal aorta is approximately 2 cm; it increases with age. An abdominal aneurysm is usually defined as an aortic diameter of 3 cm or greater. Most abdominal aortic aneurysms (AAAs) arise from below the level of the renal arteries.[2]

Thoraco-abdominal aneurysms extend to a variable degree from the thoracic aorta into the abdominal aorta and may affect the origins of the visceral and renal arteries, which must be re-implanted into the graft during repair.

Aortic aneurysms may be associated with aneurysms elsewhere - eg, in an iliac artery or popliteal artery.[2]

Large and life-threatening AAAs are preceded by a long period of subclinical growth in the diameter of the aneurysm (about 1-6 mm/year on average). The larger the AAA, the higher its growth rate and greater the risk of rupture. The risk of rupture is also greater for women.[2]

Distal embolisation may produce features of limb ischaemia. The appearance of micro-embolic lower limb infarcts in a patient with easily palpable pedal pulses suggests a popliteal or abdominal aneurysm.

Ureterohydronephrosis can also occur with AAA.

Inflammation or retroperitoneal fibrosis can complicate AAA and may cause symptoms - eg, back pain, weight loss and symptoms related to entrapment of adjacent structures.[7, 8]

Ruptured AAA

NB: the diagnosis may not be obvious. Ruptured AAA should be considered in any patient with hypotension and atypical abdominal symptoms. Similarly, abdominal pain in a patient with a known aneurysm or pulsatile mass must be considered as a possible ruptured or rapidly expanding aneurysm and treated accordingly.

Ruptured AAA may present with:

Pain in the abdomen, back or loin - the pain may be sudden and severe.

Syncope, shock or collapse:

The degree of shock varies according to the site of rupture and whether it is contained - eg, rupture into the peritoneal cavity is usually dramatic, with death before reaching hospital; whereas rupture into the retroperitoneal space may be contained initially by a temporary seal forming.

Ultrasound is simple and cheap; it can assess the aorta to an accuracy of 3 mm. It is used for initial assessment and follow-up

CT provides more anatomical details - eg, it can show the visceral arteries, mural thrombus, the 'crescent sign' (blood within the thrombus, which may predict imminent rupture) and para-aortic inflammation. CT with contrast can show rupture of the aneurysm.

MRI angiography may be used. This is safer than conventional angiography, as it does not use nephrotoxic contrast medium.

Management of uncomplicated abdominal aortic aneurysm

For uncomplicated AAA, small aneurysms (<5.5 cm) are generally monitored and larger ones (5.5 cm or greater) should be considered for surgery. There is no strong evidence to demonstrate any advantage of immediate repair for small AAAs.[9]

Medical management

Regular ultrasound monitoring - the frequency dictated by the diameter of the aneurysm at the time of detection - for example:

Surgery

Indications:

For all patients with aneurysms of 5.5 cm diameter or greater, consider elective surgical repair (if fit enough for surgery).

The decision about surgical intervention is based on the risk of surgery versus rupture for each patient. In general, the risk of rupture is mainly determined by the aneurysm diameter but rupture rates are higher in patients who smoke, in females, in those with hypertension and in those with a strong family history.

Other indications for surgery are rupture, rapid expansion, or onset of sinister symptoms/signs such as back or abdominal pain or tenderness.

Types of repair for AAA

Surgical (open) repair:

This is the traditional operation. It involves exposure of the abdominal aorta, aortic and iliac clamping and replacement of the aneurysmal segment with a prosthetic graft. The graft is effective and durable.

Endovascular aneurysm repair (EVAR) involves introducing a stent-graft system through the femoral arteries, which relines the aneurysm, diverts blood flow through the endograft and allows the aneurysm to thrombose. This requires that the aneurysm should have an adequate (1.2 cm) 'neck' below the renal arteries for stent fixation.

Currently, about 65% of patients with AAA are suitable for endovascular repair.

Pros and cons of endovascular repair:

Advantages:

Avoids open abdominal surgery.

Avoids aortic cross clamping.

Reduced mortality in the first four years of follow-up; longer-term results are unknown.[3]

Disadvantages:

Follow-up with ultrasound or CT scans is essential to monitor the endograft.

Failure of the endograft can occur - this was a common problem with early grafts but the newer designs are more durable.

Randomised controlled trials (RCTs) suggest reduced mortality in the first four years of follow-up. However, rates of death from any cause appear to equalise over the years, due to stent graft-related complications.[13, 14]

A systemic review found no long-term survival benefit for patients who have endovascular repair compared with open repair for AAA. There was also a significantly higher risks of reintervention and aneurysm rupture after endovascular repair.[15]

Prognosis

Natural history

The risk of rupture is mainly determined by aneurysm diameter.

The natural history of small AAA is gradual expansion at an annual rate of approximately 10% of the initial arterial diameter.

People with aortic aneurysms are at increased risk of cardiovascular events, mostly unrelated to the aneurysm.[2]

Elective repair

Overall mortality in the UK is 2.4% for elective AAA repair.[16]There is an inverse relationship between operative mortality and the number of cases performed in individual hospitals, with many specialist centres reporting mortality rates well below 52%.[16]

The mortality rate of aneurysm repair depends on the patient's fitness for surgery and the morphology of the aneurysm.

Patients with severe cardiorespiratory or renal disease may have high peri-operative mortality rates, and for them the threshold for elective repair may be set at a larger aneurysm diameter.

Without surgeryThe annual survival rate is only 20% for aneurysms larger than 5 cm.

Ruptured AAA

Risk of rupture increases with aneurysm size and aneurysms over 6 cm have a 25% annual risk of rupture.

The outcome is poor, with approximately 80% overall mortality from ruptured AAA.

Most patients die before reaching hospital. Surgical repair of ruptured AAA has a mortality of around 50%.

Screening by ultrasound is feasible to allow early diagnosis. The idea is to offer a single scan to men aged 65. If negative, this effectively rules out AAA for life. The roll-out of the NHS AAA Screening Programme in England began in 2009. See the reference link at the end of the heading for this section for more information on UK screening programmes.

Article Information

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions.
Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy.
Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.